Without a doubt, the found footage sub-genre receives amongst others the most hate, in my opinion in comparison to other horror genres. Movies such as 'Paranormal Activity', and 'Grave Encounters' off the top of my head being punished by a number of reviews consistently shaming the 'quality' of the movie, just because it's something that they dislike. I have to say that these two are some of my favourites in the Found Footage sub genre, my all time favourites being 'V/H/S 1&2', and 'REC 1&2'.
Now, to get to what really makes a specific movie great, or not:
Whilst movies such as 'REC' may not seem believable, they kind of are. Instead of looking at the typical look of most zombie films being spread mainly by biting - we get to see a movie looking at the origin of this virus, and instead really making the most out of the situation that it is placed in. I feel as though this makes it without a doubt one of the greatest found footage series out there. Some movies such as 'Paranormal Activity' really do shy away from the believability aspect of them, but that's what makes them all the better.
I feel that a limitation on the weapons that people are carrying really makes for a more sinister aspect within the movie that you're watching. Especially in movies that aren't filmed in areas with such loose laws on Firearms, and general weapons, an example of this could be France, or the United Kingdom. We can see that only those that are trained to use specific firearms will have them, and it will need for something to be really happening before those that do have, and are trained to be using firearms will show up to one of these places that the scene of the film will be happening. We can see that looking at films such as 'Grave Encounters' weapons are if not used at all, or are very limited in their use. I feel as though this is something that makes the movie all the more horror worthy, giving the watcher a real feel for the character's well being.
Varied Camera Use
After seeing a number of different films, an example of a film that uses only one camera would be 'Blair Witch Project' - whilst being one of the, if not the first found footage movie, it could have been made a whole lot better by including a number of different things, whilst now - forests have been used to death, this is of no fault of their own (...well maybe it is) - but I think that being able to capture numerous different character's states of mind at specific points in the movie gives for a real feel, and being able to get to know certain characters better, other than the protagonist. A great example of this being 'As above, so below'.
Whilst there could be a whole load of other examples, these are what I believe are some of the best things that come to mind to allow for an amazing movie.